For establishing a method of evalulating quantitatively the brittle fracture strength in structural stress concentrated regions, a series of researches are being conducted. In the researches so far carried out, a study intended for “through the thickness flaws” has been progressed, mainly with mild steels. In actual structures, however, various kinds of steel plates are used, and in many cases, the “part-through the thickness flaws” are the object of evaluation. In this research, a study similar to the one explained in the previous report was carried out about 80 kgf/mm² high strength steel which differs largely, in plastic deformation characteristics, as compared with the mild steel, and furthermore, consideration is made on a strength evaluation method for the surface notch, as a typical example of the “part-through the thickness flaws”.